Heat-peelable pressure-sensitive adhesive sheets comprising a substrate and a pressure-sensitive adhesive layer containing a foaming agent (for example, heat-expandable microspheres) or an inflating agent formed thereon have conventionally been known (see, for example, JP-B-50-13878, JP-B-51-24534, JP-A-56-61468, JP-A-56-61469 and JP-A-60-252681; the term “JP-B” as used herein means an “examined Japanese patent publication”, and the term “JP-A” as used herein means an “unexamined published Japanese patent application”). These heat-peelable pressure-sensitive adhesive sheets, which establish both adhesiveness and peelability after using, are characterized in that the adhesive force is lowered by foaming or inflating the foaming agent or the like by heating so that they can be easily peeled off from an adherend. Due to this characteristic, these sheets have been used in, for example, temporary fixation in the production step of electronic parts and labels to be recycled.
With the recent tendency toward small-sized and lightweight electronic instruments, attempts have been made to downsizing and chip formation of electronic parts (condensers, LEDs, etc.) to be loaded thereon. The heat-peelable pressure-sensitive adhesive sheets are used in temporary fixation of these electronic parts in the step of cut processing. In association with the downsizing as described above, the area to be adhered per chip is also lessened and, in its turn, it becomes important to ensure a sufficient effective contact area of a chip with a pressure-sensitive adhesive sheet so as to avoid adhesion failures such as chip-scattering or chipping. However, the conventional heat-peelable pressure-sensitive adhesive sheets suffer from considerable irregularity on the surface caused by particles having relatively large diameter among heat-expandable microspheres contained in the pressure-sensitive adhesive layer and, therefore, an efficient contact area between a pressure-sensitive adhesive sheet and a chip cannot be secured, which sometimes results in chip-scattering or chipping.